It is known to determine antibodies using hemagglutination to visualize their presence in a biological medium; for this purpose, erythrocytes are used which bear an antigen which reacts with the antibodies to be determined; however, the treated erythrocytes have poor stability, and this is a major disadvantage of this type of method, which is in other respects useful because it does not demand the use of expensive measuring apparatus. In French Patent A-2,331,352, a means is described for improving, in particular, the stability of the hemagglutination reagent, consisting in treating the erythrocytes, suspended in a buffer, with a lower aliphatic aldehyde and a water-soluble chromium salt before they are brought into contact with the chosen antigen; the storage period is then improved if the treated erythrocytes are stored at 4.degree. C. in a colloidal solution or in the lyophilized state. In French Patent A-2,475,737, there is a description of another means for improving the stability of red cells treated for use as a reagent in hemagglutination reactions, and which simultaneously decreases the phenomena of spontaneous agglutination; whole blood is subjected to the action of an aldehyde, such as formaldehyde or glutaraldehyde, and the red cells are then isolated and treated with a tannin before they are brought into contact with the chosen antigen to obtain a reagent which will be stored in lyophilized form.